The corrosion of automobile bodies by the salts which are sprinkled on road surfaces to prevent their freezing during the wintertime has become a big social problem recently in North America, northern Europe, and other countries or regions having a cold winter. The use of a coated steel sheet having an improved corrosion resistance instead of a conventional cold rolled steel strip has, therefore, been increasing as one of the measures for improving the corrosion resistance of automobile bodies.
Examples of the coated steel sheets are organic composite coated steel sheet, as disclosed in Japanese Patent Applications laid open under Nos. Sho 64-8033 and Hei 2-15177. These steel sheets comprise a steel sheet plated with zinc or a zinc alloy, and coated with a first layer formed by a chromate coating, and a second layer formed thereon by an organic coating composed of an organic resin which comprises a base resin obtained by adding one or more basic nitrogen atoms and two or more primary hydroxyl groups to the ends of molecules or an epoxy resin, a polyisocyanate compound and a blocked isocyanate compound, the organic resin film further containing specific proportions of silica and a sparingly soluble chromate. These strips are excellent in corrosion resistance, weldability, anti-powdering property and paint adhesion.
The recent inclination toward high-class and original automobiles has come to require a greater importance to be placed on the quality of a paint-finished body surface, particularly its image clarity. The organic composite coated steel sheet as hereinabove described has, however, the drawback of being somewhat inferior to the conventional zinc or a zinc alloy plated steel sheet in image clarity as desired of the outer surface of the outer panel of the automobile body, though it may have a good perforation corrosion resistance as required of the inner surfaces of the outer and inner panels of the automobile body. Moreover, the known organic composite coated steel sheet cannot necessarily be said to be capable of forming a satisfactorily cosmetic corrosion resistance outer surface on the outer panel of the automobile body.
Under these circumstances, it is an object of this invention to provide an organic composite coated steel sheet which can form an excellent cosmetic corrosion resistance and image clarity on, for example, the outer surface of the outer panel of an automobile body, and a process for manufacturing the same.